system: Remove uses of the 'title' field of <file-system>.

* gnu/system/install.scm (installation-os): Remove uses of the 'title'
field of 'file-system'; use 'file-system-label' as appropriate.
* gnu/system/vm.scm (system-disk-image, system-qemu-image): Likewise.
* gnu/tests.scm (%simple-os): Likewise.
* gnu/tests/install.scm (%minimal-os, %minimal-extlinux-os)
(%minimal-os-on-vda, %separate-home-os, %separate-store-os)
(%raid-root-os, %encrypted-root-os, %btrfs-root-os): Likewise.
* gnu/build/shepherd.scm (default-mounts)[tmpfs]: Likewise.
* tests/guix-system.sh: Likewise.
* tests/system.scm (%root-fs): Likewise.
("operating-system-boot-mapped-devices, implicit dependency"): Likewise.

gnu/tests.scm

@@ -208,8 +208,7 @@ the system under test."
                  (bootloader grub-bootloader)
                  (target "/dev/sdX")))
     (file-systems (cons (file-system
-                          (device "my-root")
-                          (title 'label)
+                          (device (file-system-label "my-root"))
                           (mount-point "/")
                           (type "ext4"))
                         %base-file-systems))